LTC7149 problems

Question

i have some problems with the LTC7149EUFD#PBF. 
 I am using this regulator to convert 24V input to -5V output. I designed the schematic based on the datasheet and i simulated the schematic with the LTSPICE. It worked well. 
 But during testing, when i gave an input of 5V, the obseved output was -1.026V. When the input gradually increased to 24V, the output did not change, still at -1.062V. 
 I measured the waveform of the output voltage (VOUT-) at the startup of the regulator, and I inserted the schematics and the measured output response waveform in this post. 
 Could you please give me some comments on this?

ADIApproved · Accepted Answer

please make sure the exposed pad solder to PCB GND net)

Fil · Answer

Hi jeff_jiang , 
 The VOUT went slightly positive before going down. That's not expected. Can you try isolating the load to see if this going positive is gone by using resistive load instead of an E-load? It may also help if you probe the VIN and SW together the output to see what's going on with the switching. You may also want to see if there is current limiting going on if you can probe the inductor current. 
 PCB layout plays very important role in making a good switching power supply so please follow the PCB guidelines. We would always recommend to start with our available demo boards in website and modify to your design especially to those new in power applications designs. 
 We have other inverting regulators too not buck-derived inverting regulators like ADP5073, ADP5074-75 that you may also want to take a look at.

jeff_jiang · Answer

Thanks to your suggestion. I have read the data sheet throughly, and found the exposed pad should be connected to SVOUT-, instead of GND. I separated the exposed pad of the regulator from the PCB pad with a thin layer of mica, and the regulator is working well.